In these years, fuel cells have drawn attention as a technology for the prevention of global warming. Unlike heat engines, fuel cells are believed to be able to convert chemical energy directly into electrical energy, achieve high electric power generation efficiency, and reduce energy consumption.
Solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs) are known as fuel cells capable of generating electrical power on a large scale. These fuel cells have high reaction temperatures, but may use natural gas, coal gas, and CO (carbon monoxide) gas in addition to hydrogen gas as fuel.
Therefore, in SOFCs, attempts have been made to employ hydrocarbon gas and CO gas generated from coal and hydrocarbons such as methane (CH4) gas generated by fermenting organic waste as source gases.
On the other hand, such SOFCs or MCFCs have CO2 (carbon dioxide) contained in their exhaust gas. Accordingly, various proposals have been made to improve energy efficiency and reduce CO2 emission in order to prevent global warming due to CO2 emission.
For example, Japanese Laid-open Patent Publication No. 04-065066 proposes reusing CO2 included in exhaust gas by reducing CO2 to a hydrocarbon or alcohol by electrolysis or photoelectrolysis. Japanese Laid-open Patent Publication No. 2004-192824 proposes the technique of reusing CO2 by reducing CO2 to a hydrocarbon such as methane using bacteria such as methanogen.